1. Field of Invention
The present invention relates to the field of treatment devices for low concentration waste gas, and more particular to a biofilter system for treating malodorous or low concentration organic waste gas.
2. Description of Related Arts
In the recent years of industrial production, the consumption of organic fuel has been dramatically increased, which relatively produces large amount of malodorous gases and volatile organic compounds (VOCs). These VOCs not only increases in volume and variety, but also emits in different sites. Most of these VOCs are characterized with a pungent smell, and are the cause of adverse environmental damage such as atmospheric pollution, crop failures, and the destruction of the forest, in addition to being harmful to humans. It is no wonder why VOCs have aroused a widespread concern.
There are a number of technologies for controlling these malodorous gases or VOCs, such as absorption, adsorption, incineration, condensation, and biological methods. Relatively, the biological method for treating malodorous gases or VOCs has several advantages such as being cost-effective, environmentally friendly, adaptable to a wide range of applications, and low operation cost. With the aforementioned advantages of biological methods for treating low-concentration and high flow waste gases which contain biologically biodegradable VOCs it is no wonder why it is considered one of the best methods. Gas bio-filtration is a method of gas biological filtration technology achieving the goal of purifying VOCs by oxidizing the VOCs, wherein the microorganisms attached to the packing medium play a dominant role.
When the waste gas containing low concentration of VOCs passes through the gas biological filtering reactor, its gaseous pollutants are transferred into a biofilm, and then degraded and decomposed into biomass and inorganic substances, such as CO2 and H2O, by microorganisms (enzymes).
Recently, there are mainly two types of biofilters for treating malodorous gases or VOCs, i.e. conventional biofilters and biotrickling filters. The conventional biofilter comprises a biofilter medium bed (biologically active packing medium layer), a gravel layer, and a multi-tube distributing pipeline. The biofilter medium bed is filled with a biologically active medium, such as soil, peat, sawdust, or chaff, exists microorganisms which are able to degrade pollutants, and the waste gas is purified when it passes through the packing medium layer. The biofilter medium bed has several advantages, such as short residence time, high removal efficiency, simple device and low investment and operation costs. However, it has following disadvantages, such as large footprint, uneven air distribution, and unstable performance over a long operation duration. Special difficulties associated with this process are medium bed clogging, increased drop of pressure and deterioration in the removal caused by high degree of clogging when the conventional biofilter has been run for a long time.
The biotrickling filter is also known as a trickling filter bed. It utilizes inert raw materials for the packing medium such as crushed stones, plastic particles, ceramic and carbon fibers, wherein the circulating nutrient solution sprays from top to bottom to seed and cultivate biofilms on the surface of the packing medium. The biofilm has capabilities of adsorption and bio-degradation such that the gaseous pollutants are transferred into the biofilm and degraded. Generally, it is necessary to periodically supply nutrients necessary for microorganisms to maintain long-term stable removal efficiency. Compared with the conventional biofilter, the reaction conditions of the biotrickling filter are easy to control, the concentration of microorganisms is high per unit volume of medium bed, the waste gas doesn't have to be moistened, and it can be supplied with a concurrent flow or countercurrent flow to drop filtrate flow for the biotrickling filter. Whereas, the biotrickling filter has several disadvantages, such as medium clogging resulted by excess accumulation of microorganisms in the medium bed, an increased drop of pressure caused by generation of channeling flows, and consequent decrease of performance due to excess biomass accumulation when the biotrickling filter is operated at especially under high load for a long period of time.
Existing biofilters employing gas biofiltration technology have several disadvantages: its medium bed are relatively high, its volume load is relatively low, its handling capacity is relatively small, its gas distribution is uneven, its drop of pressure is relatively large, and its packing medium bed is easy to clog. Practitioners have developed a tubular gas biofilter system which is comprised of a thin tubular layer of inert packing medium fixed in a cylindrical casing by a simple supporting means so that the cylindrical casing is divided into an outer gas chamber and an inner gas chamber. The waste gas passes through the packing medium from inside to outside or vice versa so as to degrade pollutants thereof. The nutrient solution sprays from top to bottom to supply the biofilm in the packing medium portion with nutrients. The tubular gas biofilter system definitely solves certain disadvantages of conventional biofilters and biotrickling filters, and has several advantages, such as its drop of pressure is relatively small, its height of bed layer is relatively low, its load per unit volume is high, its handling capacity is large and its operation cost is low. But tubular gas biofilters still have several disadvantages: the packing medium per unit volume of the reactor has a small surface such that its degradation efficiency is easily impacted by concentration of pollutants of the inlet gas, it has a weak shock resistance, and it has an unstable long-term performance of operation.